F9 and hemophilia B: In factor IX (FIX) exon 5, ExSpeU1s rescued exon-skipping mutations at the 5'ss and at the polypyrimidine tract with a complete recover of the functional factor IX activity.[9] Mutations is FIX exon 5 are associated to Hemophilia B, a rare X-linked hemorrhagic disorder (1/35000 males) with reduced levels of factor FIX, a key coagulation protein of liver origin.[16] The level of FIX antigen or clotting activity in the plasma determines the variability of the disease severity.